Plot

The crew of scientific ocean floor station
"Deepstar Six", funded by the Navy, is expected to perform
scientific studies as well as install an underwater nuclear missile
silo. While checking on the area they are assigned to create the
silo, they discover an underwater cave created through undersea
magma which is supposedly millions of years old. Crew's marine
biologist, Dr. Scarpelli (Nia Peeples) wants to perform further
studies into the sea cave because mysterious reports of undersea
activity in the area appeared since the 1800s. Scarpelli warns her
superior, Dr. Van Gelder (Marius Weyers) that they're supposed to
follow protocol and, if such geological discoveries are made, they
are to be reported to the Navy; however Dr. Van Gelder doesn't wish
to lose funding and, as they are very short on time with the silo,
decides to continue the project by destroying the cave. After
Osborne (Ronn Carroll) and Hodges (Thom Bray) blast the cave open, they explore
it with an unmanned vehicle, but mysteriously lose it while
scanning; they trace the pod in the cave in a small submarine when
they are attacked by a mysterious being. Joyce Collins (Nancy
Everhard) loses contact and tells Snyder (Miguel Ferrer) to get help, but before
Snyder manages to contact Van Gelder, the station is attacked again
by the mysterious creature and is damaged and slowly gets flooded,
gradually sliding towards the rift in the ocean floor; the contact
is lost. Commander of the station, Captain Laidlaw (Taurean
Blacque) and McBride (who shares romantic relationship with
Collins, unaware that she actually expects his baby) go to rescue
them, but are nearly attacked by the same creature. Only McBride
and Joyce make it out; Laidlaw gets his backbone crushed by a
falling door inside the station and commits suicide by flooding the
station.

Facing the deaths of four crew members (Hodges and Osborne in
the mini-sub; Laidlaw and Burciaga (Elya Baskin) in the flooded station) and
loss of equipment, the higher command gives green light for
abandoning the station. First they have to secure the missiles; as
Cpt. Laidlaw is dead, Snyder (presumably a military supervisor on
behalf of the Navy) is ordered to make a report and follow
procedure according to the computer instructions. Snyder reports
the situation caused by an aggressor and the computer orders the
missiles be detonated for safety. Snyder follows the orders without
question; the resulting shock wave badly damages Deepstar Six
station and the crew is forced to abandon it (its nuclear reactor -
main power source - is damaged beyond repair and will soon get
supercritical and explode), but can't because they need to
decompress before getting to the surface; unfortunately, the
decompression chamber is also damaged by the shock wave. The crew
figures out a means to restore pressure to safely leave the
station, but while Richardson is sent to repair external damage he
is attacked by the mysterious sea creature, revealed to be a huge
reptilian/crustacean with lobster-like mouth, crab claws and a
reptilian tail. Richardson manages to get back into the air lock
but before he can get out of his diving suit, water gushes into the
station and the creature gets inside, ripping Richardson in half.
Scarpelli (who was earlier shown having romantic relationship with
Richardson), shocked by his bloody demise, is attacked next and the
remaining crew runs to higher ground to regroup and arm themselves
against the creature. Snyder is given a pressurized harpoon while
the rest of the crew use shotguns. They have to complete specific
repairs to the station to leave and so they re-enter the water
filled station area armed and intentionally throw lights to
distract the creature (who was earlier seen to move towards the
light sources; Richardson was attacked when he was welding with the
electric arc underwater). When the creature emerges from the water,
Van Gelder inadvertently backs into Snyder's compressed gas spear
weapon and is punctured with a carbon dioxide charge, which
explodes, rupturing his heart.

Snyder at this point has become mentally unstable as his anxiety
and guilt has made him erratic. He panics when he hallucinates a
dead Van Gelder returning for him. Dr. Norris gives him some drugs
to calm his nerves, but it isn't enough; he suffers from complete
emotional breakdown and he runs to the only one emergency escape
pod. Snyder leaves Deepstar Six without having his body adjusted to
surface pressure; he "explodes" from the pressure during rapid
ascent. Meanwhile Dr. Norris, McBride, and Joyce are stuck in the
station and McBride risks swimming to the mini-sub in complete
darkness and silence, to use it as an escaping pod. The creature
manages to enter the room with Dr.Norris and Collins and attack
them. Joyce manages to escape to the decompression chamber; Dr.
Norris distracts the creature; tells Joyce to leave (she knows
Collins expects McBride's baby) and tries to take the creature with
her, putting defibrillator paddles in the water and administering
strong electric shock; she dies in the process and the creature is
supposedly also killed.

After Collins and McBride finish decompression, they leave in a
mini-sub a few minutes before the reactor explodes. As they reach
the surface they inflate a life raft but the creature suddenly
bursts from the ocean and attacks them. McBride distracts the
creature while Joyce stays on their emergency raft. McBride ejects
the mini-sub's fuel and ignites it with a flare gun. The sub
explodes, killing the creature. Joyce first thinks that McBride
died, but he survived by diving into the ocean under the exploding
sub and they start rowing across the ocean, waiting for the navy
rescue team to arrive.